Grapevine Varieties, Trellis Systems, and Mechanization of the California Raisin Industry

Growers in California’s San Joaquin Valley produced >25% of the world’s raisins in 2012, with a farm-gate value of >$590 million, making the United States the leading global producer of raisins. California’s traditional raisin-making method in is a laborious process in which clusters of grapes (Vitis vinifera) are harvested by hand onto paper trays that are left in the vineyard to dry. The drying fruit may need to be turned or rolled, tasks requiring manual labor, and the trays of dried raisins are also picked up by hand. Most California raisins continue to be made in this way but in recent years the declining availability and increasing cost of labor has prompted many growers to implement one of two mechanized production systems, “continuous tray” (CT), or “dry-on-vine” (DOV). In CT systems, machines are used to pick the berries, lay them onto a tray, and pick up the dried raisins. The CT system could be considered a short-term strategy: it is compatible with existing conventional ‘Thompson Seedless’ raisin vineyards, and has been widely adopted. The DOV system could be considered a medium-term strategy: it is best suited for vineyards specifically designed for DOV, with early-ripening grapevine varieties on expansive trellis systems, which ensure timely drying, and capitalize on the fact that sunlit row middles are not needed for fruit drying. Grapevine breeding programs are currently working toward the development of raisin grape varieties with fruitful basal nodes, and whose fruit dry naturally upon ripening. This is a long-term strategy to further reduce labor needs by enabling mechanical pruning in winter, and eliminating the need for cane severance in the summer.